I agree with Jeff's advice. I have an Insinkerator of similar age and
it also had a high temp thermal fuse that failed, just after the
warranty had expired. It's easy to diagnose.
I got a replacement from I think Insinkerator or someone they directed
me to. The replacement one is a different design and installs
slightly different, leading me to believe this is a known design flaw.
Which means you would think they would just send you a replacement for
free, but I think I paid about $10 for it, including shipping.
The main thing is, if it's not leaking, the cost to repair it whether
the above, a heating element, etc, should be worthwhile. The main
terminal failure mode is when it leaks.